Summary Taste contributes to quality of life, as well as to healthy eating habits. In humans and other vertebrate animals, taste transduction occurs in taste buds, when taste stimuli interact with mature (terminally differentiated) specialized cells that are often classified as type I, II, and III cells. Normal continuous turnover of taste cells underlies the homeostasis of taste; when turnover is disrupted, taste is likewise disrupted. Taste cells are derived from stem/progenitor cells, which continuously supply new taste cells throughout an animal's life. However, the mechanisms of taste cell turnover are poorly understood. Our recent studies have revealed that the Pou2f3 transcription factor is necessary for type II cell generation, and that the generation of type II cells, which mediate sweet, umami (savory), and bitter tastes, is intimately related with that of type III cells, which mediate sour and salty tastes and may be involved in intercellular lateral communication between type II and III cells. Our preliminary findings indicate that Ascl1, a transcription factor expressed in basal precursor and differentiated type III cells within taste buds, is involved in the generation of type III cells and probably type II cells in part during embryonic development in mice. We therefore propose to elucidate the roles of Ascl1 in molecular and cellular mechanisms in the generation and functional differentiation/maintenance of type III cells during taste cell turnover in adult mice. We will also examine Ascl1's role in the generation of type II cells. Because conventional Ascl1 knockout mice die shortly after birth, we will use two types of Ascl1 conditional knockout (cKO) mice. These mice have a ?floxed? Ascl1 allele that has loxP sites at 5' and 3' of an Ascl1 coding sequence, and Cre recombinase expressed in specific cell types and driving Cre-loxP mediated Ascl1 deletion in these cells. Because Ascl1 may play different roles in basal precursor and differentiated type III cells, we will conduct experiments of Aims 1 and 2 to distinguish between these roles. In Aim 1, we will induce Ascl1 cKO in differentiated type III cells. This experiment will test the hypothesis that Ascl1's expression in type III cells is necessary for functional differentiation/maintenance of these cells. In Aim 2, we will induce Ascl1 cKO in taste progenitor/stem cells that give rise to basal precursor cells. This experiment will test the hypothesis that Ascl1 is necessary for the generation of type III and II cells. In both Aims, we will analyze whether Ascl1 deficiency affects molecular and functional characteristics of specific types of taste cells, using on the taste system will be assessed in these two types of Ascl1 cKO mice with (a) histochemical gene and protein expression analyses to examine histochemical analyses and gustatory nerve recordings. Knowledge obtained from this research will improve understanding of taste cell turnover and the mechanisms of taste loss accompanying radio- and chemotherapy, infectious conditions, and aging.